In operating a blast furnace, a lower-rate operation may sometimes be carried out which comprises decreasing the production from that in the usual operation. Such a lower-rate operation can be conducted by decreasing the volume of blast blown as a reducing gas into the blast furnace through the tuyeres thereof. In other words, a decreased volume of air blast fed to the blast furnace leads to a decelerated reduction reactions of iron ore, thus resulting in a decreased production of pig iron.
However, when the volume of reducing gas flowing through the blast furnace decreases to below a certain limit through the above-mentioned decrease in the volume of blast, the flow of the blast in the blast furnace becomes non-uniform in the cross-sectional direction of the furnace. More particularly, the distribution of burden raw materials such as iron ore and coke charged into the blast furnace is not uniform, but there are portions rich and poor in voids between pieces of burden raw materials. Therefore, when the volume of blast flowing in the blast furnace decreases to below a certain limit, the blast passes through only the portions rich in voids between pieces of burden raw materials, and not through the portions poor in voids between pieces of burden raw materials. As a result, the reduction reactions of iron ore in the blast furnace cannot proceed uniformly, resulting in the furnace conditions becoming unstable and production of deposits onto the furnace wall. Such deposits onto the furnace wall, if produced, causes partial stagnation of the burden descent through the blast furnace in the form of the phenomenon known as hanging, which in turn causes slips in which broken hanging results in sudden descent of the burden. Consequently, the descending speed of burden through the blast furnace becomes non-uniform and the furnace conditions are extremely deteriorated.
A lower-rate operation of pig iron in a blast furnace is possible also by discontinuing blowing of the above-mentioned blast to be blown through the tuyeres into the blast furnace periodically for a prescribed period of time. However, when blowing of blast to be blown into the blast furnace is discontinued, the burden in the furnace is not heated during the blowoff. The interior of the blast furnace is therefore cooled, and this may result in the impossibility of discharging pig iron and slag from the blast furnace. Blowoff also causes unstable furnace conditions.
With a view to solving the above-mentioned problems, the following methods are known, which comprises, in a method for operating a blast furnace, adding to air for blast to be blown into the blast furnace nitrogen gas as the inert gas not contributing to the reduction of iron ore:
(1) a method for operating a blast furnace, disclosed in Japanese Patent Publication No. 19,481/72 filed June 3, 1972, which comprises:
to a blast to be blown into tuyeres of a blast furnace, adding nitrogen gas in an amount of up to 40 vol. % of said blast; and,
(2) a method for blowing N.sub.2 gas into a blast furnace, disclosed in Japanese Patent Provisional Publication No. 76,104/78 filed July 6, 1978, which comprises:
directing N.sub.2 gas containing at least 3% O.sub.2 discharged from an oxygen generating plant into a blast main connected to the inlet of a blower for a blast furnace, and blowing said N.sub.2 gas into the blast furnace, together with a blast.
According to the above-mentioned methods (1) and (2), by blowing a blast enriched with nitrogen gas in a prescribed volume per unit period of time into the blast furnace through the tuyeres, the reduction reactions of iron ore are decelerated even the volume of blast blown into the blast furnace is the same as the volume of blast in an operation not applied with a lower-rate production. As a result, this method allows a decrease in pig iron production while preventing furnace conditions from becoming unstable.
The above-mentioned methods (1) and (2) are however disadvantages in that they invalve higher production costs of pig iron because of the high price of nitrogen gas to be added to the air for blast. These methods are also disadvantageous in that the increase in the nitrogen content in the blast largely reduces the calorific value of blast furnace gas recovered from the top thereof. Since the aforementioned blast furnace gas is utilized as a fuel for heating furnaces in an iron works, the decrease in the calorific value of blast furnace gas adversely affects the operation of the iron works as a whole.
It has been the usual practice, on the other hand, in the operation of a blast furnace, to inject an auxiliary fuel such as heavy oil as a fuel and a reducing agent, through the tuyeres, together with the blast, with a view to reducing the consumption of coke per ton of produced pig iron (the coke rate). Recently, however, petroleum oils such as heavy oil are in short supply and prices thereof are only increasing. It is therefore becoming more common not to inject such an auxiliary fuel as heavy oil as mentioned above as fuel and reducing agent in the operation of a blast furnace, but employ coke only.
In the conventional operation with injection of an auxiliary fuel such as heavy oil, the auxiliary fuel blown through the tuyeres is decomposed at the portion near the tuyere noses into such reducing gases as carbon monoxide and hydrogen which reduce iron ore. The decomposing reaction of the above-mentioned auxiliary fuel such as heavy oil at the portion near the tuyere exits, being an endothermic reaction, causes decrease in the temperature at the portion in the blast furnace near the tuyere exits. In the conventional blast furnace operation, therefore, the portion around the tuyere exits is kept at an appropriate temperature by the temperature decreasing effect of the above-mentioned endothermic reaction. However, in the operation based on coke only without employing an auxiliary fuel such as heavy oil as mentioned above as fuel and reducing agent, an endothermic reaction does not take place at the portion around the tuyeres as that caused by an auxiliary fuel such as heavy oil as mentioned above. The temperature of the portion around the tuyere exits therefore becomes excessively high, resulting in unstable furnace conditions. The furnace conditions becoming unstable under the effect of temperature rise at the portion around the tuyere exits are attributable to the fact that the ash in the coke burnt in the portion around the tuyere exits causes production of slag rich in silicon oxide (SiO.sub.2) which floats up in a gaseous form and condenses at the bosh of the furnace, thus impairing gas permeability through the blast furnace.
Therefore, when carrying out the operation based on coke only without using an auxiliary fuel such as heavy oil, it is necessary to reduce the temperature of the portion around the tuyere exits in the blast furnace. For this purpose, the following methods have been applied to reduce the temperature of the portion around the tuyere exits:
(1) decreasing the temperature of the blast to be blown through the tuyeres into the blast furnace; and,
(2) adding steam to the blast to be blown through the tuyeres into the blast furnace, thereby causing an endothermic reaction between steam and coke at the portion around the tuyere exits, and decreasing the temperature of the portion around the tuyere exits under the effect of this endothermic reaction.
In the above-mentioned methods, however, the calorific value for reducing iron ore in the blast furnace is decreased either by the reduction of the blast temperature or by the endothermic reaction caused by steam at the portion around the tuyere exits. It becomes therefore necessary to compensate the above-mentioned decrease in the calorific value by increasing the quantity of coke charged into the blast furnace. When carrying out the operation based on coke only without using an auxiliary fuel such as heavy oil, therefore, there would be such difficulties as a considerable increase in the coke consumption per ton of pig iron (coke rate) and the rise in production costs of pig iron.
Even when operation is conducted by injecting an auxiliary fuel such as heavy oil, an operation carried out with the quantity of injected auxiliary fuel decreased as compared with that in an ordinary operation involves the same problems as in the above-mentioned operation without using an auxiliary fuel.
For these reasons, there has been a keen demand for the development of a method for operating a blast furnace, which does not lead to deteriorated furnace conditions, permits prevention of the iron production costs from increasing, and does not cause decrease in the calorific value of blast furnace gas recovered from the furnace top, when conducting a lower-rate operation of pig iron, or an operation based on coke only without using an auxiliary fuel such as heavy oil as fuel and reducing agent, or an operation with a decreased quantity of such an auxiliary fuel as heavy oil. However, such a method for operating a blast furnace is not as yet proposed.